1. Field of Invention
This invention relates generally to screw-type masonry anchors for fastening an attachment to masonry, the anchor being turned into a hole drilled in the masonry to mechanically retain the anchor therein, and more particularly to an anchor of this type having multiple lead cutting threads which enhance the holding power of the anchor.
2. Status of Prior Art
In order to fasten fixtures and other attachments to masonry, it is known for this purpose to use a screw-type anchor, such as the anchors disclosed in the Giannuzzi U.S. Pat. Nos. 5,118,496 and 5,282,708. In anchors of this type, a helical cutting thread is formed on the shank of the anchor which acts to cut and tap the bank of a hole drilled in the masonry when the anchor is turned into this hole; thereby mechanically retaining the anchor therein. The resistance of this anchor to axial pull-out forces represents its holding power.
The term masonry is generic to all stone-like building materials, such as concrete and brick. As pointed out in British patent GB 2115511 A to Godsted, when masonry is exceptionally hard, such as masonry whose aggregate is formed of granite, then in order to turn a screw-type masonry anchor into a hole drilled therein, the torque required for this purpose is very high. So high, in fact, that when seeking to screw an anchor into the masonry hole, the anchor could then break.
To create an anchor capable of being screwed into a hole drilled in exceptionally hard masonry, Godsted's anchor is provided with a helical thread having a V-shaped crossectional configuration in which the flanks of the thread intersect at an included angle of 50 to 65 degrees. While this sharp thread is capable of cutting into hard masonry, it is relatively fragile and therefore may rupture when screwed into the masonry hole.
The screw-type masonry anchor disclosed in the Ernst U.S. Pat No. 3,937,119 has a sharp-crested helical thread in which notches are formed in the crests of the thread to facilitate embedment of the crests in the bank of the drilled masonry hole. Aggregate particles cut from the bank of the masonry hole by the notches in the crest are collected in a reservoir in the space between successive convolutions of the thread.
According to Ernst, because the particles are collected, they do not produce torsional friction and make it possible to turn the anchor into the hole at relatively low torque levels. Also according to Ernst, the retention of these particles in the reservoirs "would enhance the pull out force of the anchor." But Ernst does not explain how loose particles in this reservoir act to enhance the resistance of the anchor to axial pull out forces.
The Bickford U.S. Pat No. 5,531,553 discloses a self-tapping masonry anchor having a helical cutting thread surrounding the shank of the anchor. This cutting thread is formed by a pair of parallel helical ridges having a depressed groove therebetween serving to collect substrate debris removed by the cutting action.
To facilitate self-tapping, the helical land separating the successive convolutions of the parallel ridges has a width measured axially with respect to the shank of at least 4 mm and at least 50% of the land shank diameter. The helix angle of the helical ridge in Bickford is at least 10%.
In Bickford, even if particulate debris collected in the groove in the helical cutting thread were compacted therein, it would only serve to resist displacement of the helical thread with respect to the corresponding female thread cut into the bank of the hole. But it would not serve to significantly enhance the holding power of the anchor.
Also in Bickford whose cutting thread has two parallel ridges, these ridges are in close proximity to each other, with a broad land between successive convolutions of the thread. The propinquity of the ridges does not allow each individual ridge to utilize the maximum shear strength available in the masonry material
Of greater prior art interest is the Tajima U.S. Pat. No. 4,652,194 in which particulate debris produced by the cutting action of a screw-type anchor serves to enhance the holding power of the anchor. Tajima points out that prior to his invention it was known to exploit the space between successive convolutions of the thread to collect and compact therein debris produced by the cutting thread to increase the pull-out resistance of the anchor.
According to Tajima, the spaces in prior art anchors were unduly large and it was therefore not possible for these spaces between successive convolution to be completely filled up and tightly compacted with particles to wedge the anchor in the hole.
To overcome this drawback, Tajima provides a triangular indentation in the land between successive convolutions of his helical thread. This indentation is filled with particles scraped off the bank of the masonry hole by the cutting action of the thread. However, an indented land creates a relatively large space and the particles in the space may not become tightly compacted to wedge the anchor in the hole.